Creep work hardening

Austenitic steels have a number of advantages over their ferritic cousins. They are tougher and more ductile. They can be formed more easily by stretching or deep drawing. Because diffusion is slower in f.c.c. iron than in b.c.c. iron, they have better creep properties. And they are non-magnetic, which makes them ideal for instruments like electron microscopes and mass spectrometers. But one drawback is that austenitic steels work harden very rapidly, which makes them rather difficult to machine. 

Zinc is very widely used metal in a variety of environments. The strength and hardness of zinc are greater than those of tin or lead and are less than those of aluminum or copper. It is not possible to use pure zinc in applications involving applied stress due to its low creep resistance. Zinc recrystallizes rapidly after deformation at laboratory temperature and hence cannot be work-hardened at laboratory temperature. By alloying with other metals the temperature of recrystallization and the creep resistance can be increased to acceptable levels. The alloys of commercial importance are ... 

Film Hardness Polish rate is inversely proportional to the hardness of the surface of the film being polished.Mechanical damage on the film surface is also related to film hardness. Creep and work hardening of the film during CMP will alter the hardness from values measured prior to CMP. 

The yield point, work-hardening, and recovery. The yield stress, whether in a creep or a constant strain-rate experiment, is determined by the onset of dislocation mobility, usually glide. The subsequent deformation depends on the density mobile dislocations and their speed v. Provided the dislocations are distributed reasonably homogeneously in the specimen, the deformation is described by the Orowan equation... 

In specimens deformed to several percent strain (or more) at low to intermediate temperatures and stresses, where neither work-hardening nor recovery processes predominate, dislocations tend to tangle into localized walls (Kirby and McCormick 1979 McCormick 1977 McLaren et al. 1970 Morrison-Smith et al. 1976). These walls behave as optical phase objects and give rise to the deformation lamellae that are commonly observed in deformed crystals by optical microscopy (see Section 1.3 and McLaren et al. 1970). Similar walls of tangled dislocations develop in metals in the power-law-breakdown creep regime where both recovery-controlled and glide-controlled deformation mechanisms are operative (see, e.g., Drury and Humphreys 1986). 

Several points in this general treatment require further comment. In the first place we have neglected interaction between dislocations, except for the multiplication equation (8.45). One might have expected A in (8.43) to depend on the dislocation density n as in metals, where such interaction impedes dislocation motion and leads to work-hardening. This does not occur in ice. Secondly, if we consider a normal creep experiment with exponential increase of strain with time. This does not occur and Cp tends to a constant. The probable explanation is that, when the dislocation density becomes high, dislocations can climb by a diffusion mechanism (Weertman, 1957) to annihilate each other after a limited amount of motion, thus maintaining n constant. 

By 10% shear strain or before, the flow stress has reached a steady-state value in other words, the specimen is creeping at the imposed strain rate under the resulting flow stress - that is, the work-hardening and recovery rates are equal. 

Surveying the strengthening mechanisms discussed in chapter 6, we see that grain boundary strengthening is not suitable in creep applications because we need large grains as explained above. Work hardening can also not... 

The work hardening demonstrated by the slider experiments clearly shows the effect of dislocation mobility on this rock-salt-structure material, and it is not unexpected to find much evidence for a load hardness effect with a range of n values at room temperature depending on plane and direction this is shown in Table 4.3. Dislocation mobility under the influence of the stress fields found in indentation hardening also manifests itself as an indentation creep effect as shown in Figure 4.11. 

Small alloy additions are made to zinc to improve grain size, give work hardening, and improve properties such as creep resistance and corrosion resistance. There are a number of proprietary compositions available containing additions of copper, manganese, magnesium, aluminum, chromium, and titanium. 

---